A particular problem in mobile telephone systems to which the public have access is that of fraudulent misuse, the most common form of fraud being the use of stolen or cloned handsets to evade the call billing process. This represents a significant loss of revenue to system operators. An approach to this problem has been the introduction of handsets which require the insertion of a subscriber identity card containing coded information before access to the system is permitted. For example, the GSM system requires the use of a subscriber identity module (SIM) card to allow a mobile terminal to use the system services.
It will be appreciated that a subscriber identity card will have a finite lifetime and will from time to time require replacement on an occasional basis as a result of loss by the subscriber or, more commonly, on a periodic basis to allow for normal wear and tear. This provides a problem to the system operator if security is to be maintained. To prevent fraud, the system is arranged to recognise a unique card for each subscriber. However, this requirement is generally incompatible with the temporary existence of two cards during the replacement operation. One approach to this problem is described in specification No. EP-A2-0481714 and No. GB-A-2248999 which describe the use of a SIM card or smart card to control access to a mobile system. In this arrangement, the first use of a replacement card triggers deletion of the replaced card from the system records. This process however is reliant upon diligence by the subscriber in using the replacement card promptly so that the change can be put into effect.
The object of the invention is to minimise or to overcome this disadvantage.
A further object of the invention is to provide a greater degree of control by the system operator on the card replacement process.